Phased-array antennas have traditionally been composed of a group of similar, individual element antennas or radiators oriented along a line (a linear array) or in a two-dimensional plane (a planar array). These configurations have provided the ability to form, a single, directed, pencil-beam, fan beam or even multiple beams. The formation and characteristics of the beam or beams was controlled, entirely, by amplitude and phase excitations of individual element radiators in the antennas. The main beam was scanned in space by changing the phasing and excitation of individual radiating elements. The shape of the beam (its width and sidelobes) was controlled by amplitude, phasing and spacing of the radiating elements. Scanning of the beam was accomplished completely electronically.
Phased arrays have been used in many applications including electronically steered radar, shortwave broadcasting, curtain arrays, over-the-horizon radars, ionospheric modification antennas, satellite communications, broadcasting antennas, AM broadcast service antennas, etc., etc.
A linear phased-array antenna usually includes N elements, elements equally spaced some distance d apart along a geometric line. The spacing d, when represented in wavelengths, determines the number and position, spatially, of all lobes that are generated by the antenna. Those lobes include a main beam lobe, minor sidelobes and grating lobes which are exact replicas of the main beam lobe. Usually, the antenna designer wishes to reduce all lobes except the main beam lobe, since the other lobes radiate energy in undesired directions. Minor sidelobes can be reduced to very small levels by tapering the amplitude of excitation of individual radiating elements. Grating lobes, on the other hand, can be controlled only by the wavelength spacing of the elements.
For a main beam pointing broadside to the plane of the radiating elements (0 degree scan angle), the maximum theoretical spacing between elements is one wavelength before the grating lobes begin to appear in the radiation pattern. Spacings between radiating elements of under one wavelength will insure that only minor sidelobes appear and will further assure the absence of grating lobes. Spacings of a half wavelength or less will insure that grating lobes do not appear when the radiation pattern is slewed over a variety of direction angles. Array spacings are therefore chosen, in practice, to be usually between 0.5 and 1 wavelength to eliminate all grating lobes and to allow techniques of amplitude tapering to be used to control minor sidelobes.
Because of the spacing requirements described above, phased-arrays have generally been constructed to operate only over a limited frequency range. This is because the spacing in wavelengths changes in direct proportion to frequency changes.
Many phased array antenna radiating elements are elementary dipole structures that exhibit physical dimensions close to 0.5 wavelengths in extent. This restricts the minimum spacing between centers of such elements to be just slightly greater than 0.5 wavelengths, to prevent structure overlapping. If the antenna's beam is to be directed in a broadside direction (0 degree scan angle), then the maximum spacing in wavelengths must not exceed 1 wavelength, as stated above. Where the spacing of radiating elements is 0.5 wavelengths at one frequency, if the frequency is doubled the radiating element spacing becomes 1 wavelength. Such a phased-array arrangement will thus operate over a 2:1 frequency range with acceptable performance for a 0 degree scan angle. At frequencies above twice the excitation frequency, grating lobes will appear which can no longer be eliminated by amplitude tapering.
If a phased-array is to be designed to have a slewing capability (e.g., out to as much as a +/-40 degree scan angle), then the situation becomes more difficult. In such case, a grating lobe will appear when the spacing is 0.6 wavelengths or greater. For such an array (with 0.5 wavelength spacing at its lowest frequency) the phased-array will only have a 1.2:1 frequency range in order to insure no grating lobes. The use of wideband antenna elements in a phased array will allow radiation in the main beam direction over a wide band, but will also suffer from severe pattern degradation due to many extra grating lobes in unwanted directions.
There is a need to have phased-array antenna systems which exhibit frequency independent operation over a wide bandwidth, with little or no degradation of performance as a result of undesired grating lobes. In addition, such phased-arrays should exhibit constant gain and beamwidth characteristics, satisfactory impedance response and be of simple construction.
A known wideband radiating element is the log-periodic antenna which has been used widely in many different configurations. The log-periodic antenna includes a longitudinal axis along which runs a balanced feedline to a plurality of orthogonal radiating elements. The radiating elements are generally coplanar and increase in length in a logarithmic fashion from the antenna's feed end to the antenna's far end. If paired radiating elements extend from the antenna, they are generally equal in length and extend in opposite directions normal to the longitudinal axis. Each radiating element exhibits a resonant frequency within the bandwidth of the antenna. Thus, when the antenna is energized with a signal frequency that matches the resonant frequency of a radiating element, only that radiating element becomes active and emits a radiation pattern. By varying the frequency, a variety of elements along the antenna's longitudinal axis can be made active. In general, the radiating pattern of a log-periodic antenna is co-linear with the longitudinal axis of the antenna.
The prior art contains many patents detailing various aspects of log-periodic antennas. A number of those patents relate to individual antenna structures. Such disclosure can be found in U.S. Pat. Nos. 3,134,979 and 3,308,470, both to Bell; 3,271,774 to Justice; 3,355,739 to Bell et al.; 3,366,964 to Ramsay et al.; 3,369,243 to Greiser; 3,482,250 to Maner; 3,530,484 to Barbano et al. and 3,868,689 to Liu et al. Each of the aforesaid patents discloses a structure of a log-periodic antenna; a method or apparatus for mounting such an antenna; a method or apparatus for feeding such an antenna; or a use of a particular antenna structure.
Log-periodic antennas have also been employed in arrays. In U.S. Pat. No. 3,349,404 to Copeland et al., an integrated array of log-periodic antennas and their circuitry is disclosed. The circuitry is used to switch the main lobe of the antenna over a narrow range for homing purposes. In U.S. Pat. No. 3,460,150 to Mei, a broadside, log-periodic antenna is shown wherein different lengths and configurations of feedlines to radiating elements are chosen so as to insure correct phasing relationships when all antennas are fed in parallel from a single excitation source. Mei arranges his antennas in rows and files in a substantially common plane, with the files extending from a common origin and the rows being transverse and spaced apart according to a logarithmic function. Mei discloses no ground plane for use in conjunction with his radiating elements.
In U.S. Pat. No. 4,506,268 to Kuo, a pair of log-periodic antennas are arranged in an array which is rotated around a central point. In U.S. Pat. No. 4,594,595 to Struckman, individual log-periodic antennas are arranged rotationally around a central point on a flat planar disk. Each antenna has its own individual feed point and can be thought of as having a single directional beam in the direction of orientation.
None of the aforesaid prior art achieves a wide-band phased-array that enables the generation of a radiation pattern that is grating-free over a wide slew angle.
Accordingly, it is an object of this invention to provide a wide-band, phased-array antenna using log-periodic radiating elements.
It is another object of this invention to provide a wide-band, phased-array antenna that exhibits a wide slew angle without the production of grating lobes.
It is another object of this invention to provide a wide-band, phased-array antenna employing log-periodic elements that requires no physical adjustment to achieve wide-band slew operation.